Method of manufacturing photoelectric tubes



R. B. JAN 55 Nov. 25, 194?.

METHOD OF MANUFACTURING PHOTOELECTRIC TUBES 2 Sheets-Sheet 1 Filed June 25, 1940 INVENTOR V Awamr B. JANES ATTORNE R. B. JANES Nov. 25, 1947.

METHOD OF MANUFACTURING PHOTOELECTRIC TUBES Filed June 25, 1940 2 Sheets-Sheet 2 m H M m m m m M eoemr B. JANES BY Patented Nov. 25, 1947 ZAHAM METHOD OF MANUFACTURING PHOTO- ELECTRIC TUBES Robert B. Janes, Verona, N. 5., assignor toRadio Corporation of America, a corporation of Delaware Application June 25, 1940, Serial No. 342,199

. Claims.

My invention relates to improvements in the manufacture of photo-electric tubes and electron emissive cathodes and more particularly to the photosensitization of metals of the second subgroup of the fifth group of the periodic system with metals of the alkali metal group.

Metals such as antimony and bismuth have been photosensitized by evaporating one of these metals in a vacuum to form a base layer or film, oxidizing the metal film nd forming an alloy of the oxidized metal with an alkali metal as disclosed by Gorlich U. S. 2,122,860. I have found that when this method is used the photosensitivity of such a structure is not stable, the final sensitivity obtainable is quite unpredictable and the final sensitivity is considerably lower than that which may be obtained by following the teachings of my invention.

It is an object of my invention to provide a light sensitive device having a higher photosensitivity than such devices made heretofore. It is an object of my invention to provide a process of manufacturing an electron emissive electrode, wherein the process consists of a minimum number of readily control-led steps. It is a further object to provide such a method wherein any critical process may be eliminated or performed in a time sequence which enables a duplication of tubes with uniformity of results. It is also another object to provide a process wherein various steps may be repeated or alternately employed prior to any critical step to obtain a tube having high photosensitivity. It is further object to provide a method adapted to a quantity production of photo-sensitive discharge devices.

In accordance with my invention I provide a film or base of metal such as antimony, and deposit on the film prior to any oxidation, an alkali metal to form what I believe to be an alloy between the alkali metal and the antimony base or film, the alkali metal being deposited under such conditions of temperature and pressure as to be conducive to the formation of the alloy with the metal film or base. Various baking steps may be performed prior to, during, or subsequent to the deposition of the alkali metal. Following a final baking step the tube may be found to possess sufiicient phctosensitivi'ty without sub sequent treatment. These and other objects, features and advantages of my invention will be apparent and will at once suggest themselves to those skilled in the art when considered in conmotion with the following specification and drawing in which:

Figure 1 is a longitudinal View of a tube made in accordance with my invention,

Figure 2 is a view of another type of tube made in accordance with my invention, and

Figure 3 is a block diagram showing preferred and alternate processes for manufacturing tubes such as shown in Figures 1 and 2.

Referring to the drawing, Figure l, I provide an envelope 'or bulb- I enclosing an electrically conducting foundation or cathode 2 on which the photosensitive surface is formed and an anode 3 exposed to the photosensitive surface so that it may receive electrons which are liberated from this surface under the influence of light. The cathode 2 and anode 3 which are made of nickel or other base metal are preferably supported by press '4, preferably of lead glass and in which the current carrying leads to the cathode and anode are sealed. The tube shown in the drawing is provided with a'tubulation 5 through which the tube may be evacuated and a source of caesium or other alkali metal is provided either within the bulb or in an adjacent interconnecting envelope, although I prefer to provide a quantity of caesium bearing compoimd such as an activating pellet {i which is in good thermal contact with a metal tab 7 so that caesium may be liberated from the pellet by suitable heating treatment. In one modification of my manufacturing process, I provide a source of antimony or other metal within the bulb l which may beevaporated and condensed on the cathode 2 to provide a base layer or film for the photosensitive surface. Thus a source of metal such as a quantity or mass of antimony '8 is provided within the bulb l in such a position that it may be vaporized and deposited upon the cathode 2, such as on or supported by a tungsten coil e which may be heated by electric current to vaporize the antimony. Prior to vaporizing the antimony 8, however, I prefer to bake the envelope or bulb I, together with its cathode and anode structure at a temperature of approximately 400 C. for a period of time sufiicient to liberate any occluded gases from the envelope and electrode structure, the gases being evacuated through the tubulation 5. At the temperature of this baking step the antimony is still in massive form and has little tendency to vaporize due to the temperature of baking.

In accordance with my invention and following the baking step referred to above, I vaporize a metal such as antimony and deposit the metal by condensation to form athin antimony film IE on the surface of the cathode 2 exposed to the anode 3 The antimony is vaporized and condensed on the cathode 2 in a non-oxidizing atmosphere and, in accordance with my invention, is not oxidized at any time whatsoever during 3 the subsequent steps of my manufacturing method. I have found that if this film of antimony is oxidized, the resulting photosensitivity is considerably lower than that otherwise obtained and that the sensitivity from tube to tube varies over wide ranges even though the tubes are apparently made under the same conditions. Following the condensation of antimony on the foundation 2 I may bake the tube and its associated structure at a temperature of 150 C. for a period of from 4 to 10 minutes, although I have found that this baking step may be omitted with substantially no injurious effects. Following the formation of the antimony film l and the baking step, if not omitted, I immediately vaporize alkali metal from the activating pellet 6 by heating the tab 1 such as by high frequency induced currents. Further, according to my invention and during the vaporization of the alkali metal I maintain the temperature of the foundation 2 at such a temperature as to cause an alloying of the alkali metal with the antimony film I0. I have made a great many tubes of this type and I have found that when the envelope or bulb l is maintained at a temperature from room temperature to 200 0., the photosensitivity obtained indicates that such an alloy is formed. The evidence that such an alloy is formed resides in the fact that the antimony film ID has substantially no photosensitivity prior to the liberation of caesium within the bulb l. Whereas, immediately upon the liberation of caesium the photosensitivity of the antimony film increases even when the bulb l is maintained at room temperature. The phenomenon of the antimony film becoming photosensitive is believed to be due to the formation of an antimony-caseium alloy, and since the caesium condenses on the antimony film from the vapor stage, in which state it is quite hot, this formation of an alloy apparently occurs even when the bulb l is at room temperature. I have found substantially identical results can be obtained when the envelope is heated, such as in an oven during the deposition of alkali metal as shown in the following specific examples of my process. Therefore, when I refer to maintaining the film of antimony at a temperature such that the caesium diffuses into and permeates the antimony to form an alloy, I mean that the film is maintained at a temperature between room temperature and approximately 200 C. during or immediately following the vaporization of the alkali metal. While I do not know exactly what temperature the surface of the layer or film l0 attains by reason of the deposition of the hot caesium thereon, this temperature is believed to be somewhat above room temperature, and where I speak of room temperature above I am referring to the temperature of the envelope or bulb l in that no heat from an auxiliary source other than the heat applied for evaporating the caesium is imparted to the tube during this step of my method. The caesium is preferably slowly evaporated so that the quantity may be carefully controlled, the caesium vapor going to and condensing upon the cathode 2. Somewhat more uniform photosensitivity over the entire area of the cathode may be obtained by first condensing the caesium vapor on the wall of the tube and then heating the wall to re-vaporize the caesium since the liberation from the tab 1 may be quite rapid resulting in spotty photosensitivity. This evaporation and condensation may be likened unto a distillation process and is continued until the photo-electric sensitivity has reached a first maximum, following which the tube is baked in an oven to a temperature of to C. until the photosensitivity has reached a second and higher maximum. I have found some tubes to reach this second maximum following a baking of a few minutes, whereas other tubes require baking for several hours. Continued baking after a maximum sensitivity has been obtained does not harm the tube so that, after sealing off from the exhaust system, a large quantity of tubes may be baked together in a large oven for a time sufficient for all the tubes to reach maximum sensitivity. In this process there are no steps of oxidation of either the antimony film or of the caesium.

Figure 2 shows a tube made in accordance with my invention and having a semi-transparent photocathode. The envelope 20 is provided at one end with a light transparent window 2| on which the photocathode is formed. Adjacent the opposite end of the envelope I provide means to vaporize a quantity of metal such as manganese to form an electrical conducting film of metal on the window 2 I. Thus a pellet 22 of manganese may be supported within a tungsten coil 23 adapted to be heated from source of current. Surrounding the coil 23 carrying the pellet of manganese I provide a shield 24 preferably of cylindrical form with its open end facing the window 2|. In addition, I provide a similar assembly comprising a quantity of antimony 25 similarly supported by a tungsten coil 26 within a cylindrical shield 27. Following evacuation of the tube, the tungsten coil 23 is heated to vaporize the manganese which condenses on the inner surface of the window 2i to form an electrically conducting metal film 28. The resistance of this film may be determined by providing a pair of leads extending through the wall of the envelope such as a lead 29 sealed at the central portion of the window and a lead 30 adjacent the periphery of the window 2!. The lead 29 is preferably surrounded over a small area of the window 2| by an electrically conducting deposit 3| and the window 21 likewise surrounded by an electrically conducting deposit 32 connected to the lead 30. It is obvious that only one of these leads is necessary for the operation of the tube, the two being provided merely for the purpose of determining the electrical conducting properties of the semitransparent photo-cathode. The use of the two leads is desirable only when measurements of the conducting properties of the film 28 are desired. Following the deposition of the manganese to form the cathode foundation film 28, the antimony 25 is vaporized by heating the tungsten coil 23 to form a thin film of antimony 33 on the manganese film 28. The liberation of caesium within the envelope 20 may be from a source of caesium within the envelope, as described in connection with Figure 1, or from a source of caesium external to the envelope 20, the steps of liberating caesium and the baking steps being similar to the steps described in connection with the tube shown in Figure 1, it being understood that there is no oxidation of the antimony film whatsoever.

Referring to Figure 3, which is a block diagram showing preferred and alternate processes for manufacturing phototubes in accordance with my invention, the principal and preferred steps are alphabetically indicated in sequence, the alternate steps being indicated alphabetically with numerical subscripts. Thus the final product may be obtained by following steps A to D. A baking step A1 may be advantageously incorporated following the deposition of the antimony film on the cathode foundation. Likewise, following step B of vaporizing alkali metal, the alkali metal may be condensed on the bulb wall and re-vaporized as indicated at B1 and B2. Likewise, following baking step D, steps A, A1, 13, C and D may be repeated as indicated in the block diagram by D1 to De inclusive. Likewise, steps D1 to D4 may be repeated if desired, following which the final product F is obtained.

The following tabulations show sequential steps in following my method of manufacture of phototubes and are here given merely as examples. Since the time of baking and the photosensitivity obtained is given, it is believed these tabulations are self-explanatory.

Temp., Time, Sensitivity opemlm o. Min. e/L

Evaporate Sb 0. Evaporate Cs.. 30 1. Bake 150 2. 5 0.5. Evaporate Os. 5O Low. Bake 150 4. 5 2 to 4. Evaporate Sb. 30 0.75. Bake 150 5 3. Evaporate Cs 1 to 2. Bake 150 2 Rose rapidly and dropped. Evaporate Sb 30 Rose and discontinued at Mo maximum. Bake 150 4 9 to 11. Evaporate Cs 50 Rgsezrapidly and dropped Evaporate Sb. 30 B k 150 4 4 to 5.

In the above tabulation it will be noted that the evaporation of antimony and caesium are alternated. However, after making a large number of tubes I have found that the process may be simplified considerably. Thus for tubes havin a cathode surface of square inch and an antimony film thickness of 10- centimeters, I have vaporized 3 to 4 milligrams of caesium following a bake of 30 minutes at 300 C. to remove residual gas from the bulb and electrode structures. Following the vaporization of caesium the leakage between the leads of such tubes as that shown in Figure 1 is too great to accurately determine the photosensitivity, but the leakage may be eliminated by baking for a sufficient time at 150 to 175 C., provided the stem is of lead glass. Following this final baking I have obtained an average sensitivity of 30 and 69 micro-amperes per lumen for two sets of tubes of six each. On tubes where the envelope was maintained at an elevated temperature such as 150 to 200 C. during the vaporization of caesium the average photosensitivity was found to be 33 and 46 for two sets of tubes of six each. From these results it is evident that the temperature at which the envelope of the tube is maintained may be varied over wide limits and still maintain the conditions under which the caesium will diffuse into and form what I believe to be an alloy with the antimony film so that high photosensitivity may be obtained.

While I have described my invention in connection with the use of antimony, it is to be understood that I do not wish to be limited to this particular metal, since I have found arsenic and bismuth to be satisfactory as an equivalent of antimony. Therefore, while I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only certain specific applications for which my invention may be employed, it will be apparent that my invention is by no means. limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is 5 employed without departing from the scope of my invention as set forth in the appended claims.

I claim:

The method of manufacturing a photosensitive electrode which comprises depositing by evaporation in vacuum a film of a metal which will form an alloy with an alkali metal on a supporting member, depositing by evaporation in vacuum an alkali metal directly on the exposed surface of said film of metal until the photoelectric sensitivity of the electrode reaches a first maximum and baking said film and the deposited alkali metal at a temperature of 150 C. to 170 C, to photosensitize said film to a second and higher maximum photoelectric sensitivity.

2. The method of manufacturing a, photosensitive device which comprises depositing by evaporation in vacuum a film of metal selected from the group of metals consisting of antimony, arsenic and bismuth on a supporting foundation, condensing in vacuum a vaporized alkali metal on said film of metal to photosensitize said film of metal to a first maximum of hoto-sensitivity, and baking said film and the alkali metal deposited on said film at a temperature of 150 C. to 170 C. to increase the photo-sensitivity of said film of metal to a second and higher maximum.

3. The method of manufacturing a photo-tube having a glass envelope containing an electrically conducting foundation which comprises the steps of forming on said foundation by evapora- "tion and condensation in vacuum an unoxidized film of metal selected from the group of metals consisting of antimony, arsenic, and bismuth, caesiating said unoxidized film of metal by depositing caesium on it by evaporation and condensation in vacuum to attain first maximum photo-sensitivity, and baking said caesiated film to photo-sensitize it to a second and higher maximum of photo-sensitivity, said steps being performed in the absence of an oxidizing atmosphere.

4. The method of manufacturing a photo-tube having an exhausted glass bulb enclosing an electrically conducting foundation which comprises depositing by evaporation and condensation in the exhaust bulb a film of antimony on said foundation, caesiating said film by depositing caesium by evaporation and condensation in the exhausted bulb directly on said film while said foundation is at a temperature between room temperature and 200 C. and until a first maximum photo-sensitivity is obtained and baking said bulb at 150 C. to 170 C. to increase the photo-sensitivity of said caesiated film of antimony to a second and higher maximum.

5. The method of manufacturing a photo-tube having an exhausted glass bulb with a transparent window in the wall of said bulb which comprises forming a semi-transparent electrically conducting foundation on said window by evaporation and condensation of manganese in the exhausted bulb, depositing by evaporation and condensation in the exhausted bulb a film of antimony on said foundation, caesiating said film by depositing caesium by evaporation and condensation in the exhausted bulb directly on said film while said foundation is at a temperature between room temperature and 200 C. and until a first maximum photo-sensitivity is obtained, and baking said bulb at C. to C.

to increase the photo-sensitivity of said caesiated film of antimony to a second and higher maxi- UNITED STATES PATENTS Name Date Thomson Sept. 19, 1933 Sommer Mar, 5, 1940 Smith Feb. 29, 1944 Summer July 2, 1940 Gorlich July 5, 1938 

